Εικόνες σελίδας
Ηλεκτρ. έκδοση

the resemblance between the two objects would be manifest even to sense; in the very same manner that, in geometry, the similitude of two triangles is a necessary consequence of a precise correspondence in the relations of their homologous sides.*

This last observation may serve, in some measure, to justify an assertion which was already hazarded,-That the two definitions of analogy formerly mentioned, are very nearly allied to each other;in as much as it shows, by a more careful analysis than has commonly been applied to this subject, that the sensible dissimilitude be tween things of different species arises chiefly from the want of a palpable conformity in the relations of their constituent parts. Con ceive that more remote correspondence which reason or fancy traces between the parts of the one and the parts of the other, gradually to approach, nearer and nearer, to the same standard; and is evident, that, in the course of the approximation, you will arrive at that degree of manifest resemblance, which will bring them under the same generic name; till at last, by continuing this process of the imagination, the one will become a correct picture or image of the other, not only in its great outlines, but in its minutest details.

From this view of the subject, too, as well as from the former, it appears, how vague and ill-defined the metaphysical limits are which separate the evidence of analogy from that of experience; and how much room is left for the operation of good sense, and of habits of scientific research, in appreciating the justness of that authority which, in particular instances, the popular forms of speech may assign to either.

The illustrations which I have to offer of this last remark, in so far as it relates to experience, may, I think, be introduced more usefully afterwards; but the vague conceptions which are generally annexed to the word analogy, together with the prevailing prejudices against it, as a ground of philosophical reasoning, render it proper for me, before proceeding any farther, to attempt the correction of some popular mistakes connected with the use of this obnoxious


It is not necessary, for the purposes which I have at present in view, to investigate very curiously the principles which, in the first instance, dispose the mind to indulge in analogical conjectures from the known to the unknown. It is sufficient to observe, that this disposition, so far from being checked, receives additional encourage ment from habits of philosophical study; the natural tendency of these habits being only to guide it into the right path, and to teach it to proceed cautiously, according to certain general rules, warranted by experience.

The encouragement which philosophical pursuits give to this natural disposition, arises chiefly from the innumerable proofs they afford of that systematical unity and harmony of design which are every where conspicuous in the universe. On this unity of design

· See note (Q.)

is founded the most solid argument which the light of reason supplies for the unity of God; but the knowledge of the general fact on which that argument proceeds is not confined to the student of theology. It forces itself irresistibly on the thoughts of all who are familiarly conversant with the phenomena, either of the material or of the moral world; and is recognized as a principle of reasoning, even by those who pay little or no attention to its most sublime and important application.

It is well known to all who have the slightest acquaintance with the history of medicine, that the anatomical knowledge of the ancients was derived almost entirely from analogical conjectures, founded on the dissection of the lower animals ;* and that, in consequence of this, many misrepresentations of facts, and many erroneous theories (blended, however, with various important truths,) were transmitted to the physiologists of modern Europe. What is the legitimate inference to be deduced from these premises? Not, surely, that analogy is an organ of no use in the study of nature; but that, although it may furnish a rational ground of conjecture and inquiry, it ought not to be received as direct evidence, where the fact itself lies open to examination; and that the conclusions to which it leads ought, in every case, to be distrusted, in proportion as the subjects compared depart from an exact coincidence in all their circumstances.

As our knowledge of nature enlarges, we gradually learn to combine the presumptions arising from analogy, with other general principles by which they are limited and corrected. In comparing, for example, the anatomy of different tribes of animals, we invariably find, that the differences in their structure have a reference to their way of life, and to the habits for which they are destined; so that, from knowing the latter, we might be able, on some occasions, to frame conjectures a priori concerning the former. It is thus, that

"If we read the works of Hippocrates with impartiality, and apply his accounts of the parts to what we now know of the human body, we must allow his descriptions to be imperfect, incorrect, sometimes extravagant, and often unintelligible, that of the bones only excepted. He seems to have studied these with more success than the other parts, and tells us, that he had an opportunity of seeing a buman skeleton."

"Erasistratus and Herophilus, two distinguished anatomists at Alexandria, were probably the first who were authorized to dissect human bodies. Their voluminous works are all lost; but they are quoted by Galen, almost in every page."

"What Ga en principally wanted was opportunities of dissecting human bodies: for his subject was most commonly some quadruped, whose structure was supposed to come nearest to the human.”


About the year 1540, the great Vesalius appeared. He was equally laborious in reading the ancients, and in dissecting bodies; and in making the comparison, he could not but see, that many of Galen's descriptions were erroneous.-The spirit of opposition and emulation was presently roused, and many of his contemporaries endeavoured to de fend Galen at the expense of Vesalius. In their disputes they made their appeals to the human body; and thus in a few years our art was greatly improved. And Vesalius being detected in the very fault which he condemns in Galen, to wit, describing from the dissections of brutes, and not of the human body, it exposed so fully that blunder of the older anatomists, that, in succeeding times, there has been little reason for such complaint "

[ocr errors]
[ocr errors]

Introductory Lectures, delivered by Dr. William Hunter to his last course of anatomy. (London, 1784.) pp. 13, 19, 25, 40,

the form of the teeth, together with the length and capacity of the intestines, vary in different species, according to the quality of the food on which the animal is to subsist. Similar remarks have been made on the different situation and disposition of the mammae, according as the animal is uniparous, or produces many at a birth ;— on the structure and direction of the external ear, according as the animal is rapacious, or depends for security on his speed ;-on the mechanism of the pupil of the eye, according as the animal has to search for his food by day or by night,—and on various other organs in the bodily economy, when compared with the functions which they are intended to perform. If, without attending to circumstances of this sort, a person should reason confidently from the anatomy of one species to that of another, it cannot be justly said, that analogy is a deceitful guide, but that he does not know how to apply analogy to its proper purpose. In truth, the very consideration which gives to the argument from analogy its chief force, points here manifestly to the necessity of some modification of the original conclusion, suited to the diversity of the case to which it is to be applied.

It is remarked by Cuvier, that "a canine tooth, adapted to tear "flesh, was never found combined, in the same animal, with a hoof, "fit for supporting the weight of the body, but totally useless as a 66 weapon to a beast of prey." -"Hence (he observes) the rule that "every hoofed animal is herbivorous; and hence (as corollaries "from this general principle) the maxims, that a hoofed foot indi"cates grinding teeth with flat surfaces, a long alimentary canal, a "large stomach, and often more stomachs than one, with many "other similar consequences.

"The laws which regulate the relations between different sys"tems of organs, (continues this very ingenious and sound philoso"pher,) have the same influence on the different parts of the same 66 system, and connect together its different modifications, by the "same necessary principles. In the alimentary system, especially, "where the parts are large and numerous, these rules have their "most striking applications. The form of the teeth, the length, "the convolutions, the dilatations of the alimentary canal, the num"ber and abundance of the gastric liquors, are in the most exact "adaptation to one another, and have similar fixed relations to the "chemical composition, to the solid aggregation, and to the solubi"lity of the aliment; in so much that, from seeing one of the parts "by itself, an experienced observer could form conclusions tolerably "accurate, with respect to the conformation of the other parts of "the same system, and might even hazard more than random cor"jectures with respect to the organs of other functions.

"The same harmony subsists among the different parts of the system of organs of motion. As all the parts of this system act "mutually, and are acted upon, especially when the whole body of "the animal is in motion, the forms of all the different parts are "strictly related. There is hardly a bone that can vary in its sur

"faces, in its curvatures, in its protuberances, without corresponding "variations in other bones; and in this way, a skilful naturalist, "from the appearance of a single bone, will be often able to con"clude, to a certain extent, with respect to the form of the whole skeleton to which it belonged.

"These laws of co-existence (Cuvier adds) which have just been "indicated, are deduced by reasoning from our knowledge of the "reciprocal influence of the functions, and of the use of the different (6 organs of the body. Having confirmed them by observation, we "are enabled, in other circumstances, to follow a contrary route; "and, when we discover constant relations of form between particu"lar organs, we may safely conclude, that they exercise some action (6 upon one another; and we may thus be frequently led to form just "conjectures with respect to their uses.-It is, indeed, chiefly from "the attentive study of these relations, and from the discovery of "relations which have hitherto escaped our notice, that physiology "has reason to hope for the extension of her limits; and accord"ingly, the comparative anatomy of animals is to her one of the "most fruitful sources of valuable discovery."*

The general result of these excellent observations is, that the improvement of physiology is to be expected chiefly from lights furnished by analogy; but that, in order to follow this guide with safety, a cautious and refined logic is still more necessary than in conducting those reasonings which rest on the direct evidence of experience. When the ancient anatomists, without any examination of the facts within their reach, or any consideration of the peculiar functions likely to be connected with man's erect form and rational faculties, drew inferences concerning his internal frame, merely from the structure of the quadrupeds; the errours into which they fell, so far from affording any solid argument against the use of analogy when judiciously employed,-have only pointed out to their successors the necessity of a more discriminating and enlightened application of it in future; and have ultimately led to the discovery of those comprehensive Laws of the Animal Economy, which, by reconciling apparent anomalies with the consistency and harmony of one grand design, open, at every successive step of our progress, more enlarged and pleasing views of the beneficent wisdom of Nature.

This speculation might be carried farther, by extending it to the various analogies which exist between the Animal and the Vegetable kingdoms, contrasted with those characteristical peculiarities by which they are respectively adapted to the purposes for which they are destined. It is, however, of more consequence, on the present occasion, to turn our attention to the analogies observable among many of the physical processes by which different effects are ac

See the Introduction to the Lecons d'Anatomie comparée de G. Cuvier. The above translation is taken from a very interesting tract, entitled, An Introduction to the Study of the Animal Economy. (Edinburgh, 1801.)

complished, or different phenomena produced, in the system of inanimate and unorganized matter. Of the existence of such analogies a satisfactory proof may be derived, from the acknowledged tendency of philosophical habits and scientific pursuits, to familiarize the mind with the order of nature, and to improve its penetration in anticipating future discoveries. A man conversant with physics and chemistry is much more likely than a stranger to these studies to form probable conjectures concerning those laws of nature which still remain to be examined. There is a certain character or style (if I may use the expression) in the operations of Divine Wisdom; --something which every where announces, amidst an infinite variety of detail, an inimitable unity and harmony of design; and in the perception of which philosophical sagacity and genius seem chiefly to consist. It is this which bestows a value so inestimable on the Queries of Newton.*

This view of the numberless analogies displayed in that part of the universe which falls under our immediate notice, becomes more particularly impressive, when it is considered, that the same unity of design may be distinctly traced, as far as the physical researches of astronomers have extended. In the knowledge of this fact, we possess important moral lights, for which we are entirely indebted to the Newtonian school; the universal creed of antiquity having assumed as a principle, that the celestial phenomena are, in their nature and laws, essentially different from the terrestrial. The Persian Magi, indeed, are said to have laid down, as one of their maxims,-uñain SIVICO GÓC LINH TOIS xara ;†—but that no maxim could stand in more direct opposition to the tenets of the Grecian philosophers, appears sufficiently from the general strain of their physical and astronomical theories. The modern discoveries have shown, with demonstrative evidence, how widely, in this fundamental assumption, these philosophers erred from the truth; and, indeed, it was a conjecture à priori, originating in some degree of scepticism with respect to it, that led the way to the doctrine of gravitation. Every subsequent step which has been gained in astronomical science has tended more and more to illustrate the sagacity of those views by which Newton was guided to this fortunate anticipation of the truth; as well as to confirm, upon a scale which continually grows in its magnitude, the

How very deeply Newton's mind was impressed with those ideas of analogy which I have here ventured to ascribe to him, appears from his own words. "Have not the small particles of bodies certain powers, virtues, or forces, by which they act at a distance, not only upon the rays of light for reflecting, refracting, and inflecting them, but also upon one another, for producing a great part of the phenomena of nature? For it is well known that bodies act one upon another, by the attractions of gravity, magnetism and electricity; and these instances shew the tenor and course of nature, and make it not improbable but that there may be more attractive powers than these. For nature is very consonant and conform able to herself." See the 31st Query, at the end of his Optics.

In a subsequent part of this Query, he recurs to the same principle. "And thus Nature will be very conformable to herself and very simple; performing all the great motions of the heavenly bodies by the attraction of gravity which intercedes those bodies; and almost all the small ones of their particles, some other attractive and repelling powers, which intercede the particles."

[Things above have a resemblance to those below.]

« ΠροηγούμενηΣυνέχεια »